Crosslinker-Accelerator System for Polyacrylates

ABSTRACT

Controlled thermal crosslinking in a polyacrylate-based composition is accomplished by a crosslinker-accelerator system for the thermal crosslinking of polyacrylates having functional groups suitable for entering into linking reactions with epoxide groups, comprising at least 
     one substance comprising at least one epoxide group (crosslinker) and at least one substance of the general formula (I) 
       R 1 R 2 N—CR 3 R 4 —CR 5 R 6 —(CR 7 R 8 ) n —NR 9 R 10   (I)
 
     wherein
 
the radicals R 1 , R 2 , R 9  and R 10  independently of one another are a hydrogen atom or a substituted or unsubstituted alkyl or cycloalkyl radical having 1 to 8 carbon atoms or an alkylene group bonded to the main chain and having 1 to 8 carbon atoms, where at least one of the radicals R 1 , R 2 , R 9  and R 10  is not a hydrogen atom;
 
the radicals R 3 , R 4 , R 5 , R 6 , R 7  and R 8  independently of one another are a hydrogen atom or an alkyl group having 1 to 8 carbon atoms or form a 5-7-membered cycloalkylene group; and n is an integer from 0 to 4 (accelerator).

The present invention relates to the technical field of crosslinkablepolyacrylates. In particular a system is proposed for controlling thecrosslinking rate of thermally crosslinkable polyacrylates, this systembeing based on the combination of a substance containing epoxide groupswith at least one specific diamine or polyamine.

Polyacrylates are widely used for high-grade industrial applications, asadhesives, more particularly as pressure-sensitive adhesives orheat-sealing adhesives, having proved to be highly suitable for thegrowing requirements in these areas of application. For instance,pressure-sensitive adhesives (PSAs) are required to have a good initialtack, but also to meet exacting requirements in terms of shear strength.At the same time these compositions must be suitable for coating ontocarrier materials. All of this can be achieved through the use ofpolyacrylates with a high molecular weight and high polarity, and theirefficient crosslinking. Moreover, polyacrylates can be produced to betransparent and stable to weathering.

For the coating of polyacrylate compositions useful as PSA from solutionor as a dispersion, thermal crosslinking has long been state of the art.In general, the thermal crosslinker—for example a polyfunctionalisocyanate, a metal chelate or a polyfunctional epoxide—is added to thesolution or dispersion of a polyacrylate equipped with correspondingfunctional groups, the resulting composition is coated as a sheetlikefilm onto a substrate, and the coating is subsequently dried. Throughthis procedure, organic solvents, or water in the case of dispersions,are evaporated, and the polyacrylate, accordingly, is crosslinked.Crosslinking is very important for the coatings, since it gives themsufficient cohesion and thermal shear strength. Without crosslinking,the coatings would be too soft and would flow away under even a lowload. Critical to a good coating outcome is the observance of thepotlife (processing life, within which the system is in a processiblestate), which can vary greatly according to crosslinking system. If thislife is too short, the crosslinker has already undergone reaction in thepolyacrylate solution; the solution is already partly crosslinked andcan no longer be applied uniformly.

The technological operation for producing PSAs is in a state ofcontinual onward development. Motivated by more restrictiveenvironmental impositions and by rising prices for solvents, an aim isto eliminate the solvents as far as possible from the manufacturingoperation. Within the industry, therefore, there is continual growth inthe importance of melt processes (also referred to as hotmelt processes)with solvent-free coating technology for the production of PSAs. In suchprocesses, meltable polymer compositions, i.e. polymer compositionswhich at elevated temperatures enter into the fluid state withoutdecomposing, are processed. Such compositions can be processedoutstandingly from the melt state. In ongoing developments of thisprocedure, an aim is to make the production of the product compositionsas well a low-solvent or solvent-free operation.

The introduction of the hotmelt technology is imposing growingrequirements on the adhesives. Meltable polyacrylate compositions inparticular (synonymous designations: “polyacrylate hotmelts”, “acrylatehotmelts”) are being investigated very intensively for improvements. Inthe coating of polyacrylate compositions from the melt, thermalcrosslinking has to date not been very widespread, in spite of thepotential advantages of this method.

Acrylate hotmelts have to date been crosslinked primarily throughradiation-chemical methods (UV irradiation, EBC irradiation). Thisprocedure, however, is associated with a variety of disadvantages:

-   -   In the case of crosslinking by means of UV rays, only        UV-transparent (UV-pervious) layers can be crosslinked.    -   In the case of crosslinking with electron beams (electron beam        crosslinking or electron beam curing, also EBC), the electron        beams possess only a limited depth of penetration, which is        dependent on the density of the irradiated material and on the        accelerator voltage.    -   In both of the aforementioned methods, the layers after        crosslinking have a crosslinking profile, and the        pressure-sensitive adhesive layer does not crosslink        homogeneously.

The pressure-sensitive adhesive layer must be relatively thin in orderfor well-crosslinked layers to be obtainable by chemical radiationmethods. The thickness through which radiation can pass, though indeedvarying as a function of density, accelerator voltage (EBC) and/oractive wavelength (UV), is always greatly limited; accordingly, it isnot possible to effect crosslinking through layers of arbitrarythickness, and certainly not homogeneously.

Also known in the prior art are a number of processes for the thermalcrosslinking of acrylate hotmelts. In each of these processes acrosslinker is added to the acrylate melt prior to coating, and then thecomposition is shaped and wound to form a roll.

Direct thermal crosslinking of acrylate hotmelt compositions comprisingNCO-reactive groups is described in EP 0 752 435 A1. The isocyanatesused, which are free from blocking agents and are, more particularly,sterically hindered and dimerised isocyanates, require very drasticcrosslinking conditions, and so a rational technical implementationpresents problems. Under the kind of conditions which prevail onprocessing from the melt, the procedure described in EP 0 752 435 A1leads to rapid and relatively extensive crosslinking, and so coating ofthe composition onto carrier materials is difficult. In particular it isnot possible to obtain homogeneous layers of adhesive of the kind thatare needed for many technical applications of adhesive tapes.

Also prior art is the use of blocked isocyanates. A disadvantage of thisapproach is the release of blocking groups or fragments which may havean adverse effect on the technical adhesive properties. One example isU.S. Pat. No. 4,524,104 A. It describes pressure-sensitive acrylatehotmelt adhesives which can be crosslinked using blocked polyisocyanatestogether with cycloamidines or salts thereof as catalyst. With thissystem, the required catalyst, but especially substances produced suchas HCN, phenol, caprolactam or the like, may severely impair the productproperties. With this approach, moreover, there is a need often fordrastic conditions in order to release the reactive groups. Significantapplication of this approach is so far unknown and appears, furthermore,to be unattractive.

DE 10 2004 044 086 A1 describes a process for the thermal crosslinkingof acrylate hotmelts that coats a solvent-free functionalized acrylatecopolymer, which following metered addition of a thermally reactivecrosslinker has a processing life that is long enough for compounding,conveying and coating, onto a web-form layer of a further material andthen crosslinks this coating under mild conditions until the cohesionachieved is sufficient for pressure-sensitive adhesive tapes. Adisadvantage of this process is that the reactivity of the crosslinker(isocyanate) predetermines the free processing life and the degree ofcrosslinking. Isocyanate crosslinkers react in some cases even duringtheir addition; consequently, depending on the system, the gel-free timecan be very short. A composition with a sizable fraction of functionalgroups such as hydroxyl groups or carboxylic acid can in that case nolonger be applied sufficiently well. A streaky coat interspersed withgel specks and hence inhomogeneous would be the result. Another problemwhich arises is that the achievable degree of crosslinking is limited.If a higher degree of crosslinking through addition of a higher quantityof crosslinker is desired, this has disadvantages when polyfunctionalisocyanates are used. The composition would react too quickly and wouldbe coatable, if at all, only with a very short processing life and henceat very high coating speeds, which would exacerbate the problems of thenon-homogeneous coating appearance.

Crosslinking by means of polyfunctional epoxides is described in EP 1978 069 A1, it having been shown that through the use of accelerators,without which the epoxides would undergo, to all intents and purposes,no reaction with the carboxyl groups present in the polymer, the degreeof crosslinking can be adjusted independently of the crosslinkingkinetics. In order to make sure that the composition is coatable aftermelt processing, crosslinking in the extruder must take place only to asmall extent and must subsequently continue at temperatures lower thanin the extruder, in order for ideal product properties to be achieved.While the crosslinker-accelerator systems described in EP 1 978 069 A1do meet this requirement and can be used industrially, the secondarycrosslinking at room temperature was too slow. Secondary crosslinking atelevated temperatures is frequently undesirable if the PSAs have alreadybeen wound up into rolls, which may lose their shape as a result of theheat-treatment steps.

Epoxides react fundamentally only under the influence of heat, and moreparticularly only after prolonged supply of thermal energy. Knownaccelerator substances such as ZnCl₂, for example, do lead to animprovement in the reaction capacity within the temperature range ofpolymer melts, but in the absence of a supply of thermal energy from theoutside (in other words, for example, at room temperature), thereactivity of the epoxides is lost, even in the presence of theaccelerators, and so the crosslinking reaction breaks down (in otherwords, at the prevailing temperature, the accelerators no longer have anaccelerating activity). This is a problem especially when thepolyacrylates processed as a hotmelt are coated within relatively shorttime periods (several minutes) and then, in the absence of a furthersupply of heat, cooled rapidly down to room temperature or storagetemperature. Without initiation of a further crosslinking reaction itwould not be possible to achieve high degrees of crosslinking, and fornumerous fields of application of polyacrylates, such as their use asPSAs in particular, this would have the very deleterious consequence ofinadequate cohesion of the composition.

If the crosslinker system, with only thermally functioning accelerators,such as ZnCl₂, were to be introduced too early into the polyacrylatesystem (in order to achieve a sufficient degree of crosslinking), thenthe compositions would no longer be able to be homogeneously processed,more particularly compounded and coated, since they would crosslink tooquickly or would even “gel” (undergo uncontrolled crosslinking). If, onthe other hand, the accelerator causes too little activation of epoxidecrosslinking, then a very long secondary crosslinking time can beexpected or the compositions will have to be stored at hightemperatures, which is undesirable.

It is an object of the present invention to enable thermal crosslinkingof polyacrylate compositions which can be processed from the melt(“polacrylate hotmelts”), the intention being that there should be asufficiently long processing life (“potlife”) available for processingfrom the melt, especially as compared with the potlife in the case ofthe known thermal crosslinking systems for polyacrylate hotmelts. Itought at the same time to be possible to do without the use ofprotective groups, which would have to be removed again possibly byactinic radiation or other methods. Moreover, the intention is that itshould be possible to set the degree of crosslinking of the polyacrylatecomposition to a desired level, without adversely affecting theadvantages of the operating regime. Even at low temperatures, thesecondary crosslinking is to proceed rapidly to an end level.

In the text below, the polyacrylate compositions are also referred tosynonymously for short as “polyacrylates”. For the noncrosslinkedpolyacrylate compositions, the term “polymerisates” is also used, whilethe term “polymers” is used for the fully or partly crosslinkedpolyacrylate compositions.

The above object is achieved by means of a specificcrosslinker-accelerator system comprising at least one epoxide compoundand at least one specific diamine or polyamine. A first subject of theinvention, therefore, is a crosslinker-accelerator system for thethermal crosslinking of polyacrylates having functional groups suitablefor entering into linking reactions with epoxide groups, comprising

at least one substance comprising at least one epoxide group(crosslinker) and at least one substance conforming to the generalformula (I)

R¹R²N—CR³R⁴—CR⁵R⁶—(CR⁷R⁸)_(n)—NR⁹R¹⁰  (I)

in which the radicals R¹, R², R⁹ and R¹⁰ independently of one anotherare a hydrogen atom or a substituted or unsubstituted alkyl orcycloalkyl radical having 1 to 8 carbon atoms or an alkylene groupbonded to the main chain and having 1 to 8 carbon atoms, where at leastone of the radicals R¹, R², R⁹ and R¹⁰ is not a hydrogen atom; theradicals R³, R⁴, R⁵, R⁶, R⁷ and R⁸ independently of one another are ahydrogen atom or an alkyl group having 1 to 8 carbon atoms or form a5-7-membered cycloalkylene group; and n is an integer from 0 to 4(accelerator).

A crosslinker-accelerator system of this kind enables, initially,comfortable processing lives for the polyacrylate compositions, whilelater a speedy secondary crosslinking is ensured even at lowtemperatures, more particularly at room temperature. Thecrosslinker-accelerator system of the invention avoids the above-reciteddisadvantages of conventional crosslinking systems, and iscustom-tailored to the requirements of an industrially implementedoperation for producing PSAs or PSA-coated products.

As a result of the inventive combination of the stated components it ispossible to offer a thermal crosslinking process which when applied tothe processing of polyacrylate hotmelt compositions does not lead touncontrolled reactions (gelling of the composition) and leaves asufficiently long time for processing (potlife), thus making it possiblein particular to achieve a uniform and blister-free coating when thecomposition is coated out as a layer or is applied to a carrier. A veryadvantageous effect of the crosslinker-accelerator system of theinvention is that the necessary secondary crosslinking of thepolyacrylate composition after processing, more particularly after beingcoated out as a layer or applied to a carrier, proceeds rapidly, with asupply of thermal energy reduced significantly by comparison with meltproduction, in other words after cooling, without any need for actinicirradiation for this purpose. By rapid crosslinking is meant that theelastic fraction of the composition, measured by means of microsheartravel method H3, is significantly more than 25%, more preferably morethan 35%, within a week.

As a result of the crosslinker-accelerator system of the invention, thepolyacrylates are capable of undergoing further crosslinking withoutfurther thermal energy (heating) supplied actively—that is, by technicalprocess means—more particularly after cooling to room temperature (RT,20° C.) or to a temperature close to room temperature. More particularlyit is possible in this crosslinking phase to do without heating, withoutthis leading to a termination of the crosslinking reaction.

A “crosslinker” for the purposes of the invention is a substance viawhich the polyacrylate molecules are linked to form three-dimensionalstructures by formation of covalent bonds. An “accelerator” is asubstance which supports the crosslinking reaction by ensuring asignificantly increased crosslinking reaction rate in comparison to theabsence of the accelerator. This is true, of course, in the case ofcomparable reaction parameters and more particularly at temperaturesbelow the melting temperature of the polyacrylates. Within thistemperature range, the crosslinking reaction without accelerators wouldnot proceed at all or would proceed only very slowly. The accelerator,then, produces a substantial improvement in the reaction kinetics of thecrosslinking reaction. In accordance with the invention this can be donecatalytically, but also by incorporation into the reaction event.

“Polyacrylates” are polymers whose monomer basis, in terms of amount ofsubstance, consists to an extent of at least 30% of acrylic acid,methacrylic acid, acrylic esters and/or methacrylic esters, with acrylicesters and/or methacrylic esters being present at least proportionally,generally and preferably to an extent of at least 30%. Moreparticularly, a “polyacrylate” is a polymerisate which is obtainable byradical polymerization of acrylic and/or methylacrylic monomers andalso, optionally, further, copolymerisable monomers.

The crosslinker-accelerator system of the invention comprises at leastone substance which contains at least one epoxide group as crosslinker.Substances containing epoxide groups that are used are preferablypolyfunctional epoxides, these being those which have at least twoepoxide functions per molecule (i.e. are at least difunctional inrespect of the epoxide groups). They may be either aromatic or aliphaticcompounds.

Examples of suitable polyfunctional epoxides are oligomers ofepichlorohydrin, epoxy ethers of polyhydric alcohols (more particularlyethylene, propylene and butylene glycols, polyglycols, thiodiglycols,glycerol, pentaerythritol, sorbitol, polyvinyl alcohol, polyallylalcohol and the like); epoxy ethers of polyhydric phenols (moreparticularly resorcinol, hydroquinone, bis(4-hydroxyphenyl)methane,bis(4-hydroxy-3-methylphenyl)methane,bis(4-hydroxy-3,5-dibromophenyl)methane,bis(4-hydroxy-3,5-difluorophenyl)methane,1,1-bis(4-hydroxyphenyl)ethane, 2,2-bis(4-hydroxyphenyl)propane,2,2-bis(4-hydroxy-3-methylphenyl)propane,2,2-bis(4-hydroxy-3-chlorophenyl)propane,2,2-bis(4-hydroxy-3,5-dichlorophenyl)propane,2,2-bis(4-hydroxy-3,5-dichlorophenyl)propane,bis(4-hydroxyphenyl)phenylmethane, bis(4-hydroxyphenyl)diphenylmethane,bis(4-hydroxyphenyl)-4′-methylphenylmethane,1,1-bis(4-hydroxyphenyl)-2,2,2-trichloroethane,bis(4-hydroxyphenyl)-(4-chlorophenyl)methane,1,1-bis(4-hydroxyphenyl)cyclohexane,bis(4-hydroxyphenyl)cyclohexylmethane, 4,4′-dihydroxybiphenyl,2,2′-dihydroxybiphenyl, 4,4′-dihydroxydiphenyl sulphone) and also theirhydroxyethyl ethers; phenol-formaldehyde condensation products such asphenol alcohols, phenol aldehyde resins and the like; S- andN-containing epoxides (for example N,N-diglycidylaniline,N,N′-dimethyldiglycidyl-4,4-diaminodiphenylmethane) and also epoxidesprepared by customary methods from polyunsaturated carboxylic acids ormonounsaturated carboxylic esters of unsaturated alcohols, glycidylesters, polyglycidyl esters, which may be obtained by polymerization orcopolymerization of glycidyl esters of unsaturated acids, or areobtainable from other acidic compounds (cyanuric acid, diglycidylsulphide, cyclic trimethylene trisulphone and/or derivatives thereof,and others).

Examples of ethers containing epoxide groups that are very suitable inaccordance with the invention include 1,4-butanediol diglycidyl ether,polyglycerol-3 glycidyl ether, cyclohexanedimethanol diglycidyl ether,glycerol triglycidyl ether, neopentyl glycol diglycidyl ether,pentaerythritol tetraglycidyl ether, 1,6-hexanediol diglycidyl ether,polypropylene glycol diglycidyl ether, trimethylolpropane triglycidylether, bisphenol A diglycidyl ether and bisphenol F diglycidyl ether.

The crosslinker-accelerator system of the invention further comprises atleast one substance conforming to the general formula (I)

R¹R²N—CR³R⁴—CR⁵R⁶—(CR⁷R⁸)_(n)—NR⁹R¹⁰  (I)

in which the radicals R¹, R², R⁹ and R¹⁰ independently of one anotherare a hydrogen atom or a substituted or unsubstituted alkyl orcycloalkyl radical having 1 to 8 carbon atoms or an alkylene groupbonded to the main chain and having 1 to 8 carbon atoms, where at leastone of the radicals R¹, R², R⁹ and R¹⁰ is not a hydrogen atom; theradicals R³, R⁴, R⁵, R⁶, R⁷ and R⁸ independently of one another are ahydrogen atom or an alkyl group having 1 to 8 carbon atoms or form a5-7-membered cycloalkylene group; and n is an integer from 0 to 4, asaccelerator.

By “substituted” is meant that a hydrogen atom which is bonded to acarbon atom belonging to the respective radical is replaced by anotheratom or by a chemical group, such as a functional group, for example,where the group atom bonded directly to the relevant carbon atom of theradical may in particular also be a heteroatom, in other words not acarbon atom. The term “substituted alkyl radical” covers radicals whichin spite of substitution can still be identified as alkyl radicals interms of their essential nature. These include, for example, radicalsthat are halogenated in position 1, in other words on the carbon atombonded directly to the nitrogen atom. Not covered by the term“substituted alkyl radical”, in contrast, are radicals, for example,whose carbon atom in position 1 is part of a carbonyl group or of acomparable functional group, which no longer allows the radical inquestion to be classed, in terms of its essential nature, among thealkyl radicals.

“Alkylene group bonded to the main chain” means that the alkylene groupin question joins the nitrogen atom to one of those carbon atoms towhich the radicals R³ to R⁸ belong.

Suitable substances corresponding to the general formula (I) are, forexample, 3-(dimethylamino)-1-propylamine,N,N′-dimethyl-1,3-propanediamine, N,N,N′-trimethyl-1,3-propanediamine,N-methyl-1,3-diaminopropane, N,N,2,2-tetramethyl-1,3-propane-diamine,N,N′,2-trimethyl-1,3-propanediamine,(3-Amino-2-methylpropyl)dimethylamine,(1-ethyl-3-piperidinyl)methanamine, pentamethyldiethylenetriamineN,N,N′,N″,N″-penta-methyldipropylenetriamine,N,N-bis(3-dimethylaminopropyl)-N-isopropanolamine,N-(3-dimethylaminopropyl)-N,N-diisopropanolamine,N′-(3-(dimethylamino)propyl)-N,N-dimethyl-1,3-propanediamine,N,N,N′-trimethylaminoethylethanolamine. The substance corresponding tothe general formula (I) is preferably selected from the substanceslisted above.

Preferably, at least one of the radicals R¹, R², R⁹ and R¹⁰ in thegeneral formula (I) is a methyl group. More preferably, R¹ and R² and/orR⁹ and R¹⁰ are each a methyl group. This means that either both R¹ andR² or both R⁹ and R¹⁰ or all radicals R¹, R², R⁹ and R¹⁰ in the generalformula (I) are a methyl group.

The radicals R¹, R², R⁹ and R¹⁰ in the general formula (I) mayadditionally be, as substituted alkyl radicals, for example, aminoalkylgroups or hydroxyalkyl groups. A radical R¹, R², R⁹ and R¹⁰ in the formof an alkylene group bonded to the main chain is found, for example, in(1-ethyl-3-piperidinyl)methanamine.

It has additionally been found that a particularly positive effect onthe activity of the accelerator is achieved when two aminefunctionalities are bonded to one another via a O₂ bridge. Preferably,therefore, n in the general formula (I) is 0 (zero).

More preferably, n in the general formula (I) is 0 (zero) and R¹ and R²and/or R⁹ and R¹⁰ are each a methyl group.

The at least one substance corresponding to the general formula (I) is,preferably in accordance with the invention, selected fromN,N,N,N,N-pentamethyldiethylenetriamine andN′-(3-(dimethylamino)propyl)-N,N-dimethyl-1,3-propanediamine. Thesesubstances enable particularly efficient activation of bond formationvia the epoxide groups of the crosslinker.

A further subject of the present invention is a thermally crosslinkablecomposition which comprises at least one polyacrylate having functionalgroups suitable for entering into linking reactions with epoxide groups,and a crosslinker-accelerator system of the invention. This is moreparticularly a thermally crosslinkable composition which comprises atleast one polyacrylate having functional groups which are suitable forentering into linking reactions of epoxide groups,

at least one substance which comprises at least one epoxide group(crosslinker), and also at least one substance conforming to the generalformula (I)

R¹R²N—CR³R⁴—CR⁵R⁶—(CR⁷R⁸)_(n)—NR⁹R¹⁰  (I),

in which the radicals R¹, R², R⁹ and R¹⁰ independently of one anotherare a hydrogen atom or a substituted or unsubstituted alkyl orcycloalkyl radical having 1 to 8 carbon atoms or an alkylene groupbonded to the main chain and having 1 to 8 carbon atoms, where at leastone of the radicals R¹, R², R⁹ and R¹⁰ is not a hydrogen atom; theradicals R³, R⁴, R⁵, R⁶, R⁷ and R⁸ independently of one another are ahydrogen atom or an alkyl group having 1 to 8 carbon atoms or form a5-7-membered cycloalkylene group; and n is an integer from 0 to 4(accelerator).

The total fraction of substances which comprise at least one epoxidegroup (crosslinkers) is preferably 0.1%-5% by weight, more preferably0.15%-0.4% by weight, based on the pure (without additives) polyacrylateto be crosslinked. The total fraction of accelerator is preferably0.05%-5% by weight, more preferably 0.1%-1.2% by weight, based on theadditive-free polyacrylate to be crosslinked. “Pure polyacrylate to becrosslinked” means in accordance with the invention “only thepolyacrylate to be crosslinked, without any additives”. It isparticularly advantageous if the crosslinker fraction is selected so asto result in an elastic fraction of the crosslinked polyacrylates of atleast 20%. The elastic fraction is preferably at least 40%, morepreferably at least 60% (measured in each case by measurement method H3;cf. Experimental Section).

As monomers or comonomers for preparing the polyacrylate it ispreferred, accordingly, to make proportional use of functional monomerswhich are crosslinkable with epoxide groups. These are preferablymonomers with acid groups (particularly carboxylic acid, sulphonic acidor phosphonic acid groups) and/or hydroxyl groups and/or acid anhydridegroups and/or epoxide groups and/or amine groups; monomers containingcarboxylic acid groups are particularly preferred. It is especiallyadvantageous if the polyacrylate comprises copolymerized acrylic acidand/or methacrylic acid.

Further monomers which may be used as comonomers for the polyacrylate,besides acrylic and/or methacrylic esters having up to 30 C atoms, are,for example, vinyl esters of carboxylic acids comprising up to 20 Catoms, vinylaromatics having up to 20 C atoms, ethylenically unsaturatednitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 Catoms, aliphatic hydrocarbons having 2 to 8 C atoms and 1 or 2 doublebonds, or mixtures of these monomers.

The properties of the polyacrylate (pressure-sensitive adhesive;heat-sealing composition, viscoelastic non-tacky material and the like)may be influenced in particular by varying the glass transitiontemperature of the polymer, by means of different weight fractions ofthe individual monomers.

For purely crystalline systems there is a thermal equilibrium betweencrystal and liquid at the melting point T_(m). Amorphous orsemi-crystalline systems, in contrast, are characterized by thetransformation of the more or less hard amorphous or semi-crystallinephase into a softer (rubberlike to viscous) phase. At the glass point,particularly in the case of polymeric systems, there is a “thawing” (or“freezing” in the case of cooling) of the Brownian molecular motion ofrelatively long chain segments.

The transition from the melting point T_(m) (also “melting temperature”;really defined only for purely crystalline systems; “polymer crystals”)to the glass transition point T_(g) (also “glass transitiontemperature”, “glass temperature”) can therefore be considered to be afluid transition, depending on the proportion of semi-crystallinity inthe sample under analysis.

In the sense of the remarks above, when the glass transition point isstated, the reference for the purposes of this specification includesthe melting point as well—in other words, the glass transition point (orelse, synonymously, the glass transition temperature) is also understoodto include the melting point for the corresponding “melting” systems.The statements of the glass transition temperatures relate to thedetermination by means of dynamic mechanical analysis (DMA) at lowfrequencies.

In order to obtain polymers, as for example pressure-sensitive adhesivesor heat-sealing compositions, having desired glass transitiontemperatures, the quantitative composition of the monomer mixture isadvantageously selected such that, in accordance with an equation (E1)in analogy to the Fox equation (cf. T. G. Fox, Bull. Am. Phys. Soc.1956, 1, 123), the desired T_(g) value for the polymer is produced.

$\begin{matrix}{\frac{1}{T_{g}} = {\sum\limits_{n}\; \frac{w_{n}}{T_{g,n}}}} & \left( {E\; 1} \right)\end{matrix}$

In this equation, n represents the serial number of the monomers used,w_(n) represents the mass fraction of the respective monomer n (% byweight), and T_(g,n) represents the respective glass transitiontemperature of the homopolymer of each of the monomers n, in K.

It is preferred to use a polyacrylate which can be traced back to thefollowing monomer composition:

-   a) acrylic and/or methacrylic esters of the following formula

CH₂═C(R^(I))(COOR^(II))

-   -   where R^(I)═H or CH₃ and R^(II) is an alkyl radical having 4 to        14 C atoms,

-   b) olefinically unsaturated monomers having functional groups of the    type already defined for reactivity with epoxide groups,

-   c) optionally further acrylates and/or methacrylates and/or    olefinically unsaturated monomers which are copolymerizable with    component (a).

For the use of the polyacrylate as a PSA, the fractions of thecorresponding components (a), (b) and (c) are selected such that thepolymerization product more particularly has a glass transitiontemperature ≦15° C. (DMA at low frequencies).

Especially for the preparation of PSAs it is very advantageous to selectthe monomers of component (a) with a fraction from 45% to 99% by weight,the monomers of component (b) with a fraction from 1% to 15% by weightand the monomers of component (c) with a fraction from 0% to 40% byweight (the figures are based on the monomer mixture for the “basepolymer”, i.e. without additions of any additives to the completedpolymer, such as resins etc.).

For the use of the polyacrylate as a hotmelt adhesive, in other words asa material which acquires its pressure-sensitive tack only by virtue ofheating, the fractions of components (a), (b), and (c) are selected moreparticularly such that the copolymer has a glass transition temperature(T_(g)) of between 15° C. and 100° C., preferably between 30° C. and 80°C., more preferably between 40° C. and 60° C.

A viscoelastic material, which may typically be laminated on both sideswith pressure-sensitive adhesive layers, has a glass transitiontemperature (T_(g)) in particular of between −50° C. to +100° C.,preferably between −20° C. to +60° C., more preferably 0° C. to 40° C.Here again, the fractions of components (a), (b) and (c) should beselected accordingly.

The monomers of component (a) are, in particular, plasticizing and/orapolar monomers. As monomers (a) it is preferred to use acrylic andmethacrylic esters with alkyl groups consisting of 4 to 14 C atoms, morepreferably 4 to 9 C atoms. Examples of monomers of this kind are n-butylacrylate, n-butyl methacrylate, n-pentyl acrylate, n-pentylmethacrylate, n-amyl acrylate, n-hexyl acrylate, hexyl methacrylate,n-heptyl acrylate, n-octyl acrylate, n-octyl methacrylate, n-nonylacrylate, isobutyl acrylate, isooctyl acrylate, isooctyl methacrylate,and their branched isomers, such as 2-ethylhexyl acrylate or2-ethylhexyl methacrylate, for example.

The monomers of component (b) are, in particular, olefinicallyunsaturated monomers having functional groups, in particular havingfunctional groups which are able to enter into a reaction with theepoxide groups.

Preference for component (b) is given to using monomers havingfunctional groups which are selected from the group encompassing:hydroxyl, carboxyl, sulphonic acid or phosphonic acid groups, acidanhydrides, epoxides, amines.

Particularly preferred examples of monomers of component (b) are acrylicacid, methacrylic acid, itaconic acid, maleic acid, fumaric acid,crotonic acid, aconitic acid, dimethylacrylic acid,β-acryloyloxypropionic acid, trichloroacrylic acid, vinylacetic acid,vinylphosphonic acid, itaconic acid, maleic anhydride, hydroxyethylacrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate,hydroxypropyl methacrylate, 6-hydroxyhexyl methacrylate, allyl alcohol,glycidyl acrylate, glycidyl methacrylate.

As component (c) it is possible in principle to use all compounds withvinylic functionalization which are copolymerizable with component (a)and/or component (b). The monomers of component (c) can serve to adjustthe properties of the resultant PSA.

Monomers by way of example for component (c) are as follows:

methyl acrylate, ethyl acrylate, propyl acrylate, methyl methacrylate,ethyl methacrylate, benzyl acrylate, benzyl methacrylate, sec-butylacrylate, tert-butyl acrylate, phenyl acrylate, phenyl methacrylate,isobornyl acrylate, isobornyl methacrylate, tert-butylphenyl acrylate,tert-butylphenyl methacrylate, dodecyl methacrylate, isodecyl acrylate,lauryl acrylate, n-undecyl acrylate, stearyl acrylate, tridecylacrylate, behenyl acrylate, cyclohexyl methacrylate, cyclopentylmethacrylate, phenoxyethyl acrylate, phenoxyethyl methacrylate,2-butoxyethyl methacrylate, 2-butoxyethyl acrylate,3,3,5-trimethylcyclo-hexyl acrylate, 3,5-dimethyladamantyl acrylate,4-cumylphenyl methacrylate, cyanoethyl acrylate, cyanoethylmethacrylate, 4-biphenyl acrylate, 4-biphenyl methacrylate, 2-naphthylacrylate, 2-naphthyl methacrylate, tetrahydrofurfuryl acrylate,diethylamino-ethyl acrylate, diethylaminoethyl methacrylate,dimethylaminoethyl acrylate, dimethyl-aminoethyl methacrylate,2-butoxyethyl acrylate, 2-butoxyethyl methacrylate, methyl3-methoxyacrylate, 3-methoxybutyl acrylate, phenoxyethyl acrylate,phenoxyethyl methacrylate, 2-phenoxyethyl methacrylate, butyldiglycolmethacrylate, ethylene glycol acrylate, ethylene glycolmonomethylacrylate, methoxy polyethylene glycol methacrylate 350,methoxy polyethylene glycol methacrylate 500, propylene glycolmonomethacrylate, butoxydiethylene glycol methacrylate,ethoxytriethylene glycol methacrylate, octafluoropentyl acrylate,octafluoropentyl methacrylate, 2,2,2-trifluoroethyl methacrylate,1,1,1,3,3,3-hexafluoroisopropyl acrylate,1,1,1,3,3,3-hexafluoroisopropyl methacrylate,2,2,3,3,3-pentafluoropropyl methacrylate, 2,2,3,4,4,4-hexafluorobutylmethacrylate, 2,2,3,3,4,4,4-heptafluorobutyl acrylate,2,2,3,3,4,4,4-heptafluorobutyl methacrylate,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl methacrylate,dimethylaminopropyl-acrylamide, dimethylaminopropylmethacrylamide,N-(1-methylundecyl)acrylamide, N-(n-butoxymethyl)acrylamide,N-(butoxymethyl)methacrylamide, N-(ethoxymethyl)-acrylamide,N-(n-octadecyl)acrylamide, and also N,N-dialkyl-substituted amides, suchas, for example, N,N-dimethylacrylamide, N,N-dimethylmethacrylamide,N-benzylacrylamide, N-isopropylacrylamide, N-tert-butylacrylamide,N-tert-octylacrylamide, N-methylolacryl-amide, N-methylolmethacrylamide,acrylonitrile, methacrylonitrile, vinyl ethers, such as vinyl methylether, ethyl vinyl ether, vinyl isobutyl ether, vinyl esters, such asvinyl acetate, vinyl chloride, vinyl halides, vinylidene chloride,vinylidene halides, vinylpyridine, 4-vinylpyridine, N-vinylphthalimide,N-vinyllactam, N-vinylpyrrolidone, styrene, α- and p-methylstyrene,α-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene,3,4-dimethoxystyrene, macromonomers such as 2-polystyrene-ethylmethacrylate (molecular weight Mw from 4000 to 13 000 g/mol),poly(methyl methacrylate)ethyl methacrylate (Mw from 2000 to 8000g/mol).

Monomers of component (c) may advantageously also be selected such thatthey contain functional groups which support subsequentradiation-chemical crosslinking (by electron beams, UV, for example).Suitable copolymerizable photoinitiators are, for example, benzoinacrylate and acrylate-functionalized benzophenone derivatives. Monomerswhich support crosslinking by electron irradiation are, for example,tetrahydrofurfuryl acrylate, N-tert-butylacrylamide, and allyl acrylate.

The polyacrylates may be prepared by processes familiar to the skilledperson, with particular advantage by conventional radicalpolymerizations or controlled free-radical polymerizations. Thepolyacrylates may be prepared by copolymerization of the monomericcomponents using the usual polymerization initiators and also, whereappropriate, regulators (chain transfer agents), with polymerizationtaking place at the customary temperatures in bulk, in emulsion, such asin water or liquid hydrocarbons, for example, or in solution.

The polyacrylates are prepared preferably by polymerization of themonomers in solvents, more particularly in solvents with a boiling rangefrom 50 to 150° C., preferably from 60 to 120° C., using the customaryamounts of polymerization initiators, these generally being 0.01% to 5%,more particularly 0.1% to 2%, by weight (based on the total weight ofthe monomers).

Suitable in principle are all customary initiators that are familiar tothe skilled person. Examples of free-radical sources are peroxides,hydroperoxides, and azo compounds, e.g., dibenzoyl peroxide, cumenehydroperoxide, cyclohexanone peroxide, di-tert-butyl peroxide,cyclohexylsulphonyl acetyl peroxide, diisopropyl percarbonate,tert-butyl peroctoate, benzopinacol. In one very preferred procedure,use is made as radical initiator of 2,2′-azobis(2-methylbutyronitrile)(Vazo® 67™ from DuPont) or 2,2′-azobis-(2-methylpropionitrile)(2,2′-azobisisobutyronitrile; AIBN; Vazo® 64™ from DuPont).

Solvents contemplated include alcohols such as methanol, ethanol, n- andiso-propanol, n- and iso-butanol, preferably isopropanol and/orisobutanol; and also hydrocarbons such as toluene and, in particular,benzines with a boiling range from 60 to 120° C. It is possible as wellto use ketones such as, preferably, acetone, methyl ethyl ketone, methylisobutyl ketone, and esters such as ethyl acetate, and also mixtures ofsolvents of the stated type, with preference going to mixturescontaining isopropanol, particularly in amounts of 2% to 15% by weight,preferably 3% to 10% by weight, based on the solution mixture used.

The weight-average molecular weights M_(w) of the polyacrylates aresituated preferably within a range from 20 000 to 2 000 000 g/mol, verypreferably within a range from 100 000 to 1 000 000 g/mol, and extremelypreferably in a range from 150 000 to 500 000 g/mol; the figures for theaverage molecular weight M_(w) and for the polydispersity PD in thisspecification relate to the determination by gel permeationchromatography (see measurement method A2; Experimental Section). Forthis purpose it may be advantageous to carry out the polymerization inthe presence of suitable polymerization regulators such as thiols,halogen compounds and/or alcohols, in order to set the desired averagemolecular weight.

The polyacrylate preferably has a K value of 30 to 90, more preferablyof 40 to 70, as measured in toluene (1% strength solution, 21° C.). TheK value according to Fikentscher is a measure of the molecular weightand viscosity of the polymer.

Particularly suitable for the purpose of the invention are polyacrylateswhich have a narrow molecular weight distribution (polydispersity PD<4).In spite of a relatively low molecular weight, these compositions, aftercrosslinking, have a particularly good shear strength. Moreover, thelower molecular weight allows easier processing from the melt, since theflow viscosity is lower as compared with a broader-range polyacrylate,with largely the same application properties. Narrow-range polyacrylatesmay be prepared, advantageously, by anionic polymerization or bycontrolled radical polymerization methods, the latter being especiallysuitable. Examples of polyacrylates of this kind which are prepared bythe RAFT process are described in U.S. Pat. No. 6,765,078 B2 and U.S.Pat. No. 6,720,399 B2. Polyacrylates of this kind can also be preparedvia N-oxyls, as described in EP 1 311 555 B1, for example. Atom TransferRadical Polymerization (ATRP) as well can be employed advantageously forthe synthesis of narrow-range polyacrylates, in which case it ispreferred as initiator to use monofunctional or difunctional, secondaryor tertiary halides and, for abstracting the halide(s), complexes of Cu,Ni, Fe, Pd, Pt, Ru, Os, Rh, Co, Ir, Ag or Au. The various possibilitiesof ATRP are further described in specifications U.S. Pat. No.5,945,491A, U.S. Pat. No. 5,854,364 A and U.S. Pat. No. 5,789,487 A.

The polyacrylates to be crosslinked contain functional groups which aresuitable for entering into linking reactions with epoxide groups. Bylinking reactions are meant, in particular, addition reactions andsubstitution reactions. Preferably, then, there is a linking of theunits which carry the functional groups to the units which carry theepoxide groups, more particularly in the sense of a crosslinking of thepolymer units which carry the functional groups, via theepoxide-group-carrying crosslinker molecules as linking bridges. Thesubstances containing epoxide groups are preferably polyfunctionalepoxides, in other words those having at least two epoxide groups;accordingly, there is overall an indirect linking of the units whichcarry the functional groups.

The crosslinkable composition of the invention may comprise at least onetackifying resin. Tackifying resins which can be used are the existingtackifier resins described in the literature. Reference may be made inparticular to all aliphatic, aromatic and alkylaromatic hydrocarbonresins, hydrocarbon resins based on pure monomers, hydrogenatedhydrocarbon resins, functional hydrocarbon resins, and natural resins.With preference it is possible to use pinene resins and indene resins,and rosins, their disproportionated, hydrogenated, polymerized andesterified derivatives and salts, terpene resins and terpene-phenolicresins, and also C5 resins, C9 resins and other hydrocarbon resins.Combinations of these and further resins may also be used with advantagein order to adjust the properties of the resultant composition inaccordance with what is desired. With particular preference is itpossible to use all resins that are compatible (soluble) with thepolyacrylate in question. With particular preference the crosslinkablecomposition of the invention comprises terpene-phenolic resins and/orrosin esters.

The crosslinkable composition of the invention may optionally alsocomprise fillers in powder and/or granular form, likewise dyes andpigments, including more particularly abrasive and reinforcing fillerssuch as, for example, chalks (CaCO₃), titanium dioxides, zinc oxides andcarbon blacks, even in high proportions, in other words from 1% to 50%by weight, based on the overall formula. With great preference it ispossible to use various forms of chalk as a filler, with particularpreference being given to the use of Mikrosöhl chalk. At preferredfractions of up to 30% by weight, based on the overall composition, theaddition of fillers produces virtually no change in the technicaladhesive properties (shear strength at RT, instantaneous bond strengthto steel and PE).

Furthermore, fillers of low flammability, such as ammoniumpolyphosphate, for example; electrically conductive fillers such asconductive carbon black, carbon fibres and/or silver-coated beads, forexample; thermally conductive materials such as boron nitride, aluminiumoxide and silicon carbide, for example; ferromagnetic additives such asiron(III) oxides, for example; additives for increasing volume, moreparticularly for producing foamed layers or syntactic foams, such as,for example, expandants, solid glass beads, hollow glass beads,carbonized microbeads, hollow phenolic microbeads and microbeads ofother materials, expandable microballoons (Expancel® from AkzoNobel),silica, silicates, organically renewable raw materials such as woodflour, for example, organic and/or inorganic nanoparticles, fibres;ageing inhibitors, light stabilizers, ozone inhibitors, compoundingagents and/or expandants, may be present in the crosslinkablecomposition or in the fully crosslinked composition. Ageing inhibitorswhich can be used are preferably not only primary inhibitors, such as4-methoxyphenol or Irganox® 1076, but also secondary ageing inhibitors,such as Irgafos® TNPP or Irgafos® 168 from BASF, optionally also incombination with one another. Other ageing inhibitors which can be usedare phenothiazine (C radical scavenger) and also hydroquinone methylether in the presence of oxygen, and also oxygen itself.

Optionally it is possible to add customary plasticizers (plasticizingagents), more particularly in concentrations of up to 5% by weight.Plasticizers which can be metered in are, for example, low molecularweight polyacrylates, phthalates, water-soluble plasticizers,plasticizer resins, phosphates, polyphosphates, adipates and/orcitrates.

As a further possible option, the thermally crosslinkable and/orcrosslinked polyacrylate or polyacrylates may in accordance with theinvention also be blended with other polymers. Suitable for this purposeare polymers based on natural rubber, synthetic rubber, EVA, siliconerubber, acrylic rubber and polyvinyl ether. It has proven to be usefulto add these polymers in granulated or otherwise comminuted form to thepolyacrylate before the thermal crosslinker is added. The polymer blendsare produced preferably in an extruder, more preferably in a multi-screwextruder or in a planetary roller extruder. To stabilize the thermallycrosslinked polyacrylates, especially polymer blends of thermallycrosslinked acrylate hotmelts and other polymers, it may be sensible toirradiate the shaped material with a low dose of electron irradiation.For this purpose it is possible optionally to admix the polyacrylatewith crosslinking promoters such as di-, tri- or polyfunctionalacrylate, polyesters and/or urethane acrylate.

A further subject of the invention is a crosslinked polyacrylate whichis obtainable by thermal crosslinking of the thermally crosslinkablecomposition of the invention.

A further subject of the invention is a process for the thermalcrosslinking of polyacrylates having functional groups which aresuitable for entering into linking reactions with epoxide groups, saidprocess encompassing the use of a crosslinker-accelerator system of theinvention.

The process of the invention may include a concentration of thepolyacrylate solution or dispersion resulting from the polymerpreparation procedure. Concentration of the polymer may take place inthe absence of the crosslinker and accelerator substances. It is alsopossible, however, for one at most of these substances to be added tothe polymer even before concentration, in which case the concentrationtakes place in the presence of this or these substance(s).

The polymerisates are then preferably transferred to a compounder. Inspecial versions of the process of the invention, concentration andcompounding may take place in the same reactor.

The compounder used may in particular be an extruder. Within thecompounder, the polymerisates are present preferably in the melt, eitherbecause they are in the melt state when they are introduced, or byvirtue of their heating in the compounder until a melt is formed. Thepolymerisates are advantageously held in the melt in the compounder byheating.

Where neither crosslinkers (epoxides) nor accelerators are present inthe polymerisate, the possible temperature in the melt is limited by thedecomposition temperature of the polymerisate. The operationaltemperature within the compounder is typically between 80 to 150° C.,more particularly between 100 and 120° C.

The substances containing epoxide groups are added to the polymerpreferably before or with the addition of accelerator. They may be addedto the monomers even before or during the polymerization phase, if theyare sufficiently stable for that phase. With particular preference,however, the substances containing epoxide groups are added to thepolymerisate either before addition to the compounder or during additionto the compounder, in other words are introduced into the compoundertogether with the polymerisates.

In a very preferred procedure the accelerator substances are added tothe polymerisates shortly before further processing, more particularlybefore a coating or other shaping operation. The time window for theaddition prior to coating is guided in particular by the potlife whichis available, in other words the processing life in the melt, withoutdeleterious change to the properties of the resultant product. With theprocess of the invention it was possible to achieve potlives of severalminutes up to several tens of minutes (depending on the choice of theexperimental parameters), and so the accelerator ought to be addedwithin this time period prior to coating. The accelerator isadvantageously added as late as possible to the melt, but as early asnecessary, so that there is still effective homogenization with thepolymer composition.

Time periods which have emerged as being very advantageous here arethose from 2 to 10 minutes, more particularly those of more than 5minutes, before the beginning of further processing, at an operatingtemperature of 110 to 120° C.

The crosslinkers (epoxides) and the accelerators can also both be addedshortly before the further processing of the polymer, in other wordsadvantageously in the phase as set out above for the accelerators. Forthis purpose it is advantageous if crosslinkers and accelerators areintroduced into the operation simultaneously, at the same location,possibly in the form of an epoxide-accelerator blend.

In principle it is also possible to switch the times of addition and/orlocations of addition for crosslinker and accelerator in the versionsset out above, so that the accelerator may be added before thesubstances containing epoxide groups.

In the compounding operation, the temperature of the polymerisate onaddition of the crosslinkers and/or accelerators is preferably between50 and 150° C., more preferably between 70 and 130° C. and verypreferably between 80 and 120° C.

It has emerged in principle as being very advantageous for thecrosslinker, i.e. the substance containing epoxide groups, to be addedat 0.1%-5% by weight, more preferably 0.15%-0.4% by weight, based on thepolymer without additives. It is advantageous to add the accelerator at0.05%-5% by weight, more preferably at 0.1%-1.2% by weight, based on theadditive-free polymer. It is particularly advantageous if thecrosslinker fraction is selected so as to result in an elastic fractionof the crosslinked polyacrylates of at least 20%. The elastic fractionis preferably at least 40%, more preferably at least 60% (measured ineach case by measurement method H3; cf. Experimental Section).

After coating has taken place, the polymer composition cools relativelyrapidly, down to the storage temperature, generally to room temperature.The crosslinker-accelerator system of the invention is outstandinglysuitable for causing the crosslinking reaction to progress without thesupply of further thermal energy (without heat supply). The inventiveaccelerator or accelerators and accelerator concentrations mayadvantageously be selected such that the elastic fraction of thepolyacrylate after departure from the compounding operation, at lowertemperatures than in the operation, such as at room temperature, forexample, exceeds a level of 25%, preferably of 35%, within a week,preferably within three days, so that there is already a functionalproduct (more particularly an adhesive tape or a functional carrierlayer based on the polyacrylate).

The crosslinking reaction between the functional groups of thepolyacrylate and the epoxides, by means of the crosslinker-acceleratorsystem of the invention, thus proceeds even without supply of heat understandard conditions, more particularly at room temperature, tocompletion.

For stating the proportions of the constituents of thecrosslinker-accelerator system of the invention to one another or to thepolyacrylate to be crosslinked, it is possible to use the ratio of thenumber of epoxide groups in the crosslinker to the number of reactivefunctional groups in the polyacrylate. In principle this ratio can beselected freely, with either an excess of functional groups, numericalequality of the groups, or an excess of epoxide groups.

This ratio is preferably selected such that the epoxide groups are indeficit or at most in numerical equality; with very particularpreference, the ratio of the total number of epoxide groups in thecrosslinker to the number of functional groups in the polyacrylate thatare suitable for entering into linking reactions with epoxide groups isin the range from 0.01:1 to 1:1, more particularly in the range from0.02:1 to 0.4:1.

Preferably, therefore, the functional groups, more preferably carboxylicacid groups, in the polyacrylate are present in an excess over theepoxide groups, so that the polymerisate contains a sufficiently largenumber of functional groups—that is, potential crosslinking or linkingsites—in order to achieve the desired crosslinking.

The ratio of the number of acceleration-active groups in the acceleratorto the number of epoxide groups in the crosslinker may in principle beselected freely, so there is alternatively an excess ofacceleration-active groups, numerical equality of the groups, or anexcess of epoxide groups. Groups considered to be acceleration-activegroups are amino groups and phosphino groups, terms which comprehend allprimary, secondary and tertiary, and hence all substituted andunsubstituted, amino and phosphino groups. The ratio of the number ofall the substituted and unsubstituted amino and phosphino groups in theaccelerator to the number of epoxide groups in the crosslinker ispreferably from 0.2:1 to 4:1.

After the composition has been compounded, the polymer can be processedfurther, more particularly by coating onto a permanent or temporarycarrier. A permanent carrier remains joined to the layer of adhesive inthe application, whereas the temporary carrier is removed in the courseof further processing, as for example when converting the adhesive tape,or at the time of application of the layer of adhesive.

Coating of the self-adhesive compositions may take place using hotmeltcoating nozzles known to the skilled person, or, preferably, using rollapplicator mechanisms, also called coating calenders. The coatingcalenders may consist advantageously of two, three, four or more rolls.

Preferably at least one of the rolls is provided with an anti-adhesiveroll surface, this being true preferably of all of the rolls that comeinto contact with the polyacrylate. In an advantageous procedure it ispossible for all of the rolls of the calender to have an anti-adhesivefinish. An anti-adhesive roll surface used is preferably asteel-ceramic-silicone composite. Roll surfaces of this kind areresistant to thermal and mechanical loads.

It has emerged as being particularly advantageous if roll surfaces areused that have a surface structure, more particularly of a kind suchthat the surface does not produce full contact with the polymer layer tobe processed, but instead such that the area of contact is lower ascompared with a smooth roll. Particularly advantageous are structuredrolls such as engraved metal rolls—engraved steel rolls, for example.

Coating may take place with particular advantage in accordance with thecoating techniques set out in WO 2006/027387 A1 from page 12, line 5 topage 20, line 13, and more particularly as in the sections “Variant A”(page 12), “Variant B” (page 13), “Variant C” (page 15), “Method D”(page 17), “Variant E” (page 19) and also FIGS. 1 to 6.

The stated disclosure passages from WO 2006/027387 A1 are thereforeexplicitly included in the disclosure content of the presentspecification.

When coating it is possible to realise coating speeds of up to 300m/min, especially when using multi-roll calenders.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a compounding and coating operation in a continuousprocess.

FIG. 2 is a graph depicting the effect of epoxide concentration on thedegree of crosslinking for a given amount of accelerator and a giventemperature.

FIG. 3 is a graph depicting the effect of accelerator concentration fora given temperature (in this case room temperature) and with a constantamount of epoxide.

FIG. 4 shows a process for making a three-layer composition by 2-rollcalendar.

FIG. 5 is a graph depicting room temperature kinetics of differentaccelerators with the same epoxide concentrations and the same number ofbasic groups.

Shown by way of example in FIG. 1 of the present specification is thecompounding and coating operation, on the basis of a continuous process.The polymers are introduced at the first feed point (1.1) into thecompounder (1.3), here for example an extruder. Either the introductiontakes place already in the melt, or the polymers are heated in thecompounder until the melt state is reached. At the first feed point,together with the polymer, the epoxide-containing compounds areadvantageously introduced into the compounder.

Shortly before coating takes place, the accelerators are added at asecond feed point (1.2). The outcome of this is that the acceleratorsare added to the epoxide-containing polymers not until shortly beforecoating, and the reaction time in the melt is low.

The reaction regime may also be discontinuous. In correspondingcompounders such as reactor tanks, for example, the addition of thepolymers, of the crosslinkers and of the accelerators may take place atdifferent times and not, as shown in FIG. 1, at different locations.

Immediately after coating—preferably by means of roll application or bymeans of an extrusion die—the polymer is only slightly crosslinked, butnot yet sufficiently crosslinked. The crosslinking reaction preferablyproceeds predominantly on the carrier.

Crosslinking raises the cohesion of the polymer and hence also the shearstrength. The links are very stable. This allows very ageing-stable andheat-resistant products to be produced, such as adhesive tapes,viscoelastic carrier materials or shaped articles.

The physical properties of the end product, especially its viscosity,bond strength and tack, can be influenced through the degree ofcrosslinking, and so the end product can be optimized through anappropriate choice of the reaction conditions. A variety of factorsdetermine the operational window of this process. The most importantinfluencing variables are the amounts (concentrations and proportionsrelative to one another), the chemical nature of the crosslinkers andthe accelerators, the operating and coating temperatures, the residencetime in the compounder (more particularly extruder) and in the coatingassembly, the fraction of functional groups, more particularly acidgroups and/or hydroxyl groups, in the polymer, and also the averagemolecular weight of the polyacrylate.

The crosslinker-accelerator system of the invention, in processes forthe crosslinking of polyacrylates, offers the advantage that a stablecrosslinking process for polyacrylates can be offered, and one withoutstanding control facility in relation to the crosslinking pattern, byvirtue of substantial decoupling of degree of crosslinking andreactivity (reaction kinetics), more particularly the reaction kineticsat low temperatures. The amount of crosslinker (amount of epoxide) addedhere largely influences the degree of crosslinking of the product; thechemical nature and the concentration of the accelerator largely controlthe reactivity.

Surprisingly it has been observed that through the amount ofepoxide-containing substances added it has been possible to preselectthe degree of crosslinking, and to do so largely independently of theprocess parameters that typically require additional selection:temperature and amount of added crosslinker.

The effect of epoxide group concentration on the degree of crosslinkingfor a given amount of accelerator and a given temperature is shownschematically by FIG. 2. Here, the accelerator concentration rises fromthe concentration A (top curve; low concentration) via theconcentrations B (second-lowest concentration) and C (second-highestconcentration) to the concentration D (bottom curve; highestconcentration). As can be seen, the final value of the degree ofcrosslinking—represented here by increasingly smaller values for themicroshear travel—goes up as the epoxide concentration increases,whereas the reaction kinetics remain virtually unaffected.

It has also been found that the amount of accelerator added has a directinfluence on the crosslinking rate, and hence also on the point in timeat which the final degree of crosslinking is achieved, but withoutinfluencing it absolutely. The reactivity of the crosslinking reactionhere may be selected such that the crosslinking also during storage ofthe completed product under the conditions customary therein (roomtemperature) leads within a few weeks to the desired degree ofcrosslinking, more particularly without any need for thermal energy tobe (actively) supplied or for the product to be treated further.

The relationship between crosslinking time and accelerator concentrationfor a given temperature (in this case room temperature) and with aconstant amount of epoxide is reproduced schematically in FIG. 3. Here,the accelerator concentration rises from the concentration 1 (top curve;low concentration) via the concentrations 2 (second-lowestconcentration) and 3 (second-highest concentration) to the concentration4 (bottom curve; highest concentration). Here it is found that the finalvalue of the degree of crosslinking remains virtually constant (in thecase of the lowest reaction, this value has not yet been reached); withhigh concentrations of accelerator, however, this value is reached morequickly than at low concentrations of accelerator.

In addition to the aforementioned parameters, the reactivity of thecrosslinking reaction can also be influenced by varying the temperature,if desired, especially in those cases where the advantage of “inherentcrosslinking” in the course of storage under standard conditions has nopart to play. At constant crosslinker concentration, an increase in theoperating temperature leads to a reduced viscosity, which enhances thecoatability of the composition but reduces the processing life.

An increase in the processing life is acquired by a reduction in theaccelerator concentration, reduction in molecular weight, reduction inthe concentration of functional groups in the addition polymer,reduction of the acid fraction in the addition polymer, use ofless-reactive crosslinkers (epoxides) or of less-reactivecrosslinker-accelerator systems, and reduction in operating temperature.

An improvement in the cohesion of the composition can be obtained by avariety of pathways. In one, the accelerator concentration is increased,which reduces the processing life. At constant acceleratorconcentration, it also possible to raise the molecular weight of thepolyacrylate, which is possibly more efficient. In the sense of theinvention it is advantageous in any case to raise the concentration ofcrosslinker (substances containing epoxide groups). Depending on thedesired requirements profile of the composition or of the product it isnecessary to adapt the above-mentioned parameters in a suitable way.

A further subject of the invention is the use of acrosslinker-accelerator system of the invention for producing thermallycrosslinked polyacrylates.

Inventively crosslinked polyacrylates can be used for a broad range ofapplications. Below, a number of particularly advantageous fields of useare set out by way of example.

A polyacrylate crosslinked with the crosslinker-accelerator system ofthe invention is used in particular as a pressure-sensitive adhesive(PSA), preferably as a PSA for an adhesive tape, where the acrylate PSAis in the form of a single-sided or double-sided film on a carriersheet. These polyacrylates are especially suitable when a high coatweight in one coat is required, since with this coating technique it ispossible to achieve an almost arbitrarily high coat weight, preferablymore than 100 g/m², more preferably more than 200 g/m², and to do so inparticular at the same time as homogeneous crosslinking through thecoat. Examples of favourable applications are technical adhesive tapes,more especially for use in construction, examples being insulatingtapes, corrosion control tapes, adhesive aluminium tapes,fabric-reinforced film-backed adhesive tapes (duct tapes),special-purpose adhesive construction tapes, such as vapour barriers,adhesive assembly tapes, cable wrapping tapes, self-adhesive sheetsand/or paper labels.

The inventively crosslinked polyacrylate may also be made available as aPSA for a carrierless adhesive tape, in the form of what is called anadhesive transfer tape. Here as well, the possibility of setting thecoat weight almost arbitrarily high in conjunction with homogeneouscrosslinking through the coat is a particular advantage. Preferredweights per unit area are more than 10 g/m² to 5000 g/m², morepreferably 100 g/m² to 3000 g/m².

The inventively crosslinked polyacrylate may also be present in the formof a heat-sealing adhesive in adhesive transfer tapes or single-sided ordouble-sided adhesive tapes. Here as well, for carrier-containingpressure-sensitive adhesive tapes, the carrier may be an inventivelyobtained viscoelastic polyacrylate.

One advantageous embodiment of the adhesive tapes obtained using aninventively crosslinked polyacrylate can be used as a strippableadhesive tape, more particularly a tape which can be detached againwithout residue by pulling substantially in the plane of the bond.

The crosslinker-accelerator system of the invention or the crosslinkercomposition of the invention is also particularly suitable for producingthree-dimensional shaped articles, whether they be tacky or not. Aparticular advantage of this process is that there is no restriction onthe layer thickness of the polyacrylate to be crosslinked and shaped, incontrast to UV and EBC curing processes. In accordance with the choiceof coating assembly or shaping assembly, therefore, it is possible toproduce structures of any desired shape, which are then able to continuecrosslinking to desired strength under mild conditions.

This system or composition is also particularly suitable for theproduction of particularly thick layers, especially ofpressure-sensitive adhesive layers or viscoelastic acrylate layers, witha thickness of more than 80 μm. Layers of this kind are difficult toproduce with the solvent technology, since, for example, this technologyentails bubble formation and very slow coating speeds. The alternativelamination of thin layers one over another is complicated and harboursweak points.

Thick pressure-sensitive adhesive layers may be present, for example, inunfilled form, as straight acrylate, or in resin-blended form or in aform filled with organic or inorganic fillers. Also possible is theproduction of layers foamed to a closed-cell or open-cell form inaccordance with the known techniques, as well as of syntactic foams,using the crosslinker-accelerator system of the invention or thethermally crosslinkable composition of the invention. Possible methodsof foaming are those of foaming via compressed gases such as nitrogen orCO₂, or foaming via expandants such as hydrazines or expandablemicroballoons. Where expandable microballoons are used, the compositionor the shaped layer is advantageously activated suitably by means ofheat introduction. Foaming may take place in the extruder or aftercoating. It may be judicious to smooth the foamed layer by means ofsuitable rollers or release films. To produce foam-analogous layers itis also possible to add hollow glass beads or pre-expanded polymericmicroballoons to the tacky, thermally crosslinked polyacrylate.

In particular it is possible, using systems or compositions of theinvention, to produce thick layers as well, which can be used as acarrier layer for double-sidedly PSA-coated adhesive tapes. Withparticular preference these are filled and foamed layers which can beutilized as carrier layers for foamlike adhesive tapes. With theselayers as well it is sensible to add hollow glass beads, solid glassbeads or expanding microballoons to the polyacrylate prior to theaddition of the crosslinker-accelerator system or of the crosslinker orof the accelerator. It is possible to laminate a pressure-sensitiveadhesive layer onto at least one side of a foamlike viscoelastic layerof this kind. It is preferred to laminate a corona-pretreatedpolyacrylate layer on both sides. Alternatively it is possible tolaminate differently pretreated adhesive layers, i.e. pressure-sensitiveadhesive layers and/or heat-activable layers based on polymers otherthan on acrylates, onto the viscoelastic layer. Suitable base polymersare adhesives based on natural rubber, synthetic rubbers, acrylate blockcopolymers, styrene block copolymers, EVA, certain polyolefins, specificpolyurethanes, polyvinyl ethers, and silicones. Preferred compositions,however, are those which have no significant fraction of migratableconstituents and whose compatibility with the polyacrylate is so goodthat they diffuse in significant quantities into the acrylate layer andalter the properties therein.

Instead of laminating a pressure-sensitive adhesive layer onto bothsides, it is also possible on at least one side to use ahotmelt-adhesive layer or thermally activable adhesive layer. Asymmetricadhesive tapes of this kind allow the bonding of critical substrateswith a high bonding strength. An adhesive tape of this kind can be used,for example, to affix EPDM rubber profiles to vehicles.

One particular advantage of the inventively crosslinked polyacrylates isthat these layers, whether utilized as a viscoelastic carrier, as apressure-sensitive adhesive or as a heat-sealing composition, combine anequal surface quality with no crosslinking profile through the layer(or, correspondingly, through the shaped article produced from thepolyacrylates)—in particular in contrast to UV-crosslinked andEBC-crosslinked layers. As a result it is possible for the balancebetween adhesive and cohesive properties to be controlled and setideally for the layer as a whole through the crosslinking. In the caseof radiation-crosslinked layers, in contrast, there is generally oneside or one sublayer which is over- or undercrosslinked.

EXAMPLES Measurement Methods (General): K Value (According toFikentscher) (Measurement Method A1):

The K value is a measure of the average molecular size of high-polymermaterials. It is measured by preparing one percent strength (1 g/100 ml)toluenic polymer solutions and determining their kinematic viscositiesusing a Vogel-Ossag viscometer. Standardization to the viscosity of thetoluene gives the relative viscosity, from which the K value can becalculated by the method of Fikentscher (Polymer 1967, 8, 381 ff.)

Gel Permeation Chromatography GPC (Measurement Method A2):

The figures for the weight-average molecular weight M_(w) and thepolydispersity PD in this specification relate to the determination bygel permeation chromatography. Determination is made on a 100 μl samplesubjected to clarifying filtration (sample concentration 4 g/l). Theeluent used is tetrahydrofuran with 0.1% by volume of trifluoroaceticacid. Measurement takes place at 25° C. The preliminary column used is acolumn type PSS-SDV, 5μ, 10³ Å, ID 8.0 mm 50 mm. Separation is carriedout using the columns of type PSS-SDV, 5μ, 10³ Å and also 10^(5 Å) and10⁶ Å each with ID 8.0 mm×300 mm (columns from Polymer StandardsService; detection by means of Shodex R171 differential refractometer).The flow rate is 1.0 ml per minute. Calibration takes place against PMMAstandards (polymethyl methacrylate calibration).

Solids Content (Measurement Method A3):

The solids content is a measure of the fraction of non-evaporableconstituents in a polymer solution. It is determined gravimetrically, byweighing the solution, then evaporating the evaporable fractions in adrying cabinet at 120° C. for 2 hours and reweighing the residue.

Measurement Methods (PSAs): 180° Bond Strength Test (Measurement MethodH1):

A strip 20 mm wide of an acrylate PSA applied to polyester as a layerwas applied to steel plates which beforehand had been washed twice withacetone and once with isopropanol. The pressure-sensitive adhesive stripwas pressed onto the substrate twice with an applied pressurecorresponding to a weight of 2 kg. The adhesive tape was then removedfrom the substrate immediately with a speed of 300 mm/min and at anangle of 180°. All measurements were conducted at room temperature.

The results are reported in N/cm and have been averaged from threemeasurements. The bond strength to polyethylene (PE) was determinedanalogously.

Holding Power (Measurement Method H2):

A strip of the adhesive tape 13 mm wide and more than 20 mm long (30 mm,for example) was applied to a smooth steel surface which had beencleaned three times with acetone and once with isopropanol. The bondarea was 20 mm·13 mm (length·width), the adhesive tape protruding beyondthe test plate at the edge (by 10 mm, for example, corresponding toaforementioned length of 30 mm). Subsequently the adhesive tape waspressed onto the steel support four times, with an applied pressurecorresponding to a weight of 2 kg. This sample was suspended vertically,with the protruding end of the adhesive tape pointing downwards.

At room temperature, a weight of 1 kg was affixed to the protruding endof the adhesive tape. Measurement is conducted under standard conditions(23° C., 55% humidity) and at 70° C. in a thermal cabinet.

The holding power times measured (times taken for the adhesive tape todetach completely from the substrate; measurement terminated at 10 000min) are reported in minutes and correspond to the average value fromthree measurements.

Microshear Test (Measurement Method H3):

This test serves for the accelerated testing of the shear strength ofadhesive tapes under temperature load.

Sample Preparation for Microshear Test:

An adhesive tape (length about 50 mm, width 10 mm) cut from therespective sample specimen is adhered to a steel test plate, which hasbeen cleaned with acetone, in such a way that the steel plate protrudesbeyond the adhesive tape to the right and the left, and that theadhesive tape protrudes beyond the test plate by 2 mm at the top edge.The bond area of the sample in terms of height·width=13 mm·10 mm. Thebond site is subsequently rolled over six times with a 2 kg steel rollerat a speed of 10 m/min. The adhesive tape is reinforced flush with astable adhesive strip which serves as a support for the travel sensor.The sample is suspended vertically by means of the test plate.

Microshear Test:

The sample specimen for measurement is loaded at the bottom end with aweight of 100 g. The test temperature is 40° C., the test duration 30minutes (15 minutes' loading and 15 minutes' unloading). The sheartravel after the predetermined test duration at constant temperature isreported as the result in μm, as both the maximum value [“max”; maximumshear travel as a result of 15-minute loading]; and the minimum value[“min”; shear travel (“residual deflection”) 15 minutes after unloading;on unloading there is a backward movement as a result of relaxation].Likewise reported is the elastic component in percent [“elast”; elasticfraction=(max−min)·100/max].

Measurement Methods (Three-Layer Constructions): 90° Bond Strength toSteel—Open and Lined Side (Measurement Method V1):

The bond strength to steel is determined under test conditions of 23°C.+/−1° C. temperature and 50%+/−5% relative humidity. The specimenswere cut to a width of 20 mm and adhered to a steel plate. Prior to themeasurement the steel plate is cleaned and conditioned. For this purposethe plate is first wiped down with acetone and then left to stand in theair for 5 minutes to allow the solvent to evaporate. The side of thethree-layer assembly facing away from the test substrate was then linedwith a 50 μm aluminium foil, thereby preventing the sample fromexpanding in the course of the measurement. This was followed by therolling of the test specimen onto the steel substrate. For this purposethe tape was rolled over 5 times back and forth with a rolling speed of10 m/min using a 2 kg roller. Immediately after the rolling-onoperation, the steel plate was inserted into a special mount whichallows the specimen to be removed at an angle of 90° vertically upwards.The measurement of bond strength was made using a Zwick tensile testingmachine. When the lined side is applied to the steel plate, the openside of the three-layer assembly is first laminated to the 50 μmaluminium foil, the release material is removed, and the system isadhered to the steel plate, and subjected to analogous rolling-on andmeasurement.

The results measured on both sides, open and lined, are reported in N/cmand are averaged from three measurements.

Holding Power—Open and Lined Side (Measurement Method V2):

Specimen preparation took place under test conditions of 23° C.+/−1° C.temperature and 50%+/−5% relative humidity. The test specimen was cut to13 mm and adhered to a steel plate. The bond area is 20 mm·13 mm(length·width). Prior to the measurement, the steel plate was cleanedand conditioned. For this purpose the plate was first wiped down withacetone and then left to stand in the air for 5 minutes to allow thesolvent to evaporate. After bonding had taken place, the open side wasreinforced with a 50 μm aluminium foil and rolled over back and forth 2times using a 2 kg roller. Subsequently a belt loop was attached to theprotruding end of the three-layer assembly. The whole system was thensuspended from a suitable device and subjected to a load of 10 N. Thesuspension device is such that the weight loads the sample at an angleof 179°±/−1°. This ensures that the three-layer assembly is unable topeel from the bottom edge of the plate. The measured holding power, thetime between suspension and dropping of the sample, is reported inminutes and corresponds to the average value from three measurements. Tomeasure the lined side, the open side is first reinforced with the 50 μmaluminium foil, the release material is removed, and adhesion to thetest plate takes place as described. The measurement is conducted understandard conditions (23° C., 55% humidity).

Commercially Available Chemicals Used

Chemical compound Trade name Manufacturer CAS No.2,2′-Azobis(2-methylbutyronitrile) Vazo ® 67 DuPont 13472-08-72,2′-Azobis(isobutyronitrile), AIBN Vazo ® 64 DuPont 78-67-1Bis-(4-tert-butylcyclohexyl)peroxydicarbonate Perkadox ® 16 Akzo Nobel15520-11-3 Terpene-phenolic-based tackifier Dertophene ® T105 DRT,France 73597-48-5 resin (softening point 105° C., hydroxyl value 30-60)Pentaerythritol tetraglycidyl ether Polypox ® R16 UPPC AG 3126-63-43,4-Epoxycyclohexylmethyl 3,4- Uvacure ® 1500 Cytec Industries 2386-87-0epoxycyclohexanecarboxylate Inc. Dimethyl propanephosphonate Levagard ®DMPP Lanxess 18755-43-6 N,N,N,N,N-Pentamethyl- Jeffcat ® ZR-40 Huntsman3030-47-5 diethylenetriamine N′-(3-(Dimethylamino)propyl)-N,N- Jeffcat ®Z-130 Huntsman 6711-48-4 dimethyl-1,3-propanediamine DiethylenetriamineEpikare ® 3223 Hexion Spec. 111-40-0 Chemicals N,N,N,N-Tetramethyl-Sigma-Aldrich 51-80-9 methanediamine Thermoplastic hollow microbeadsExpancel ® 092 DU Akzo Nobel (particle size 10-17 μm; density max. 400.017 g/cm³; expansion temperature 127-139° C. [start]; 164-184° C.[max. exp.]) all specification figures at 20° C.; Epikare ® also soldunder the Epi-Cure ® and Bakelite ® EPH trade names

Pressure Sensitive Adhesive (PSA) Examples Preparation of StartingPolymers for Examples B1 to B4

Described below is the preparation of the starting polymers. Thepolymers investigated are prepared conventionally via free radicalpolymerization in solution.

Base Polymer P1

A reactor conventional for free-radical polymerizations was charged with30 kg of 2-ethyl-hexyl acrylate, 67 kg of n-butyl acrylate, 3 kg ofacrylic acid, and 66 kg of acetone/isopropanol (96:4). After nitrogengas had been passed through the reactor for 45 minutes with stirring,the reactor was heated to 58° C. and 50 g of2,2′-azobis(2-methylbutyronitrile) were added. Subsequently the externalheating bath was heated to 75° C. and the reaction was carried outconstantly at this external temperature. After 1 h a further 50 g of2,2′-azobis(2-methylbutyronitrile) were added, and after 4 h the batchwas diluted with 23 kg of acetone/isopropanol mixture (96:4).

After 5 h and again after 7 h, reinitiation took place with 150 g ofbis(4-tert-butylcyclohexyl) peroxydicarbonate in each case. After areaction time of 22 h the polymerization was terminated and the batchwas cooled to room temperature. The polyacrylate has a conversion of99.6%, a K value of 75.1, an average molecular weight of M_(w)=1 480 000g/mol, polydispersity PD (M_(w)/M_(n))=16.1.

Base Polymer P2 (Viscoelastic Carrier)

In the same way as in Example P1, 68 kg of 2-ethylhexyl acrylate, 25 kgof methyl acrylate and 7 kg of acrylic acid were polymerized in 66 kg ofacetone/isopropanol (94:6). Initiation was carried out twice with 50 gof 2,2-azobis(2-methylbutyronitrile) in each case, twice with 150 g ofbis(4-tert-butylcyclohexyl) peroxydicarbonate in each case, and dilutionwas carried out with 20 kg of acetone/isopropanol mixture (94:6). Aftera reaction time of 22 h the polymerization was terminated and the batchwas cooled to room temperature.

The polyacrylate has a conversion of 99.7%, a K value of 51.3 and anaverage molecular weight of M_(w)=676 000 g/mol, polydispersity PD(M_(w)/M_(n))=9.5.

Base Polymer P3 (PSA for Three-Layer Construction in Combination withViscoelastic Carrier)

In the same way as in Example P1, 24.0 kg of 2-ethylhexyl acrylate, 12.0kg of n-butyl acrylate and 4.0 kg of acrylic acid were polymerized in26.7 kg of acetone/benzine 60/95 (1:1). After nitrogen gas had beenpassed through the reactor for 45 minutes with stirring, the reactor washeated to 58° C. and 30 g of AIBN were added. The external heating bathwas subsequently heated to 75° C. and the reaction was carried outconstantly at this external temperature. After a reaction time of 1 h, afurther 30 g of AIBN were added. After 4 h and 8 h, dilution took placewith 10.0 kg each time of an acetone/benzene 60/95 (1:1) mixture. Forreduction of the residual initiators, 90 g portions ofbis-(4-tert-butylcyclohexyl) peroxydicarbonate were added after 8 h andagain after 10 h. After a reaction time of 24 h, the reaction wasterminated and the batch was cooled to room temperature.

The polyacrylate has a conversion of 99.7%, a K value of 46.9 and anaverage molecular weight of M_(w)=1 500 900 g/mol, polydispersity PD(M_(w)/M_(n))=17.0.

Process 1: Concentration/Preparation of the Hotmelt PSAs:

The acrylate copolymers (base polymers P1 and P2) are very largely freedfrom the solvent by means of a single-screw extruder (concentratingextruder, Berstorff GmbH, Germany) (residual solvent content ≦0.3% byweight; cf. the individual examples). The parameters given here by wayof example are those for the concentration of base polymer P1. The screwspeed was 150 rpm, the motor current 15 A, and a throughput of 58.0 kgliquid/h was realized. For concentration, a vacuum was applied at 3different domes. The reduced pressures were, respectively, between 20mbar and 300 mbar. The exit temperature of the concentrated hotmelt isapproximately 115° C. The solids content after this concentration stepwas 99.8%.

Process 2: Preparation of the Modified Hotmelt PSAs and ViscoelasticCarriers:

The acrylate hotmelt PSAs prepared in accordance with Process 1 aselucidated above were conveyed directly into a downstream Weldingtwin-screw extruder (Welding Engineers, Orlando, USA; model 30 mm DWD;screw diameter 30 mm, length of screw 1=1258 mm; length of screw 2=1081mm; 3 zones). Via a solids metering system, the resin Dertophene® T105was metered in zone 1 and mixed in homogeneously. In the case of thecomposition for Examples MT 1, no resin was metered in; instead, thehollow thermoplastic microbeads, mixed to a paste with Levagard® DMPPbeforehand, were metered in via the solids metering system. Theparameters given here by way of example are those for resin compoundingwith base polymer P1. The speed was 451 rpm, the motor current 42 A, anda throughput of 30.1 kg/h was realized. The temperatures of zones 1 and2 were each 105° C., the melt temperature in zone 1 was 117° C., and thecomposition temperature on exit (zone 3) was 100° C.

Process 3: Production of the Inventive Adhesive Tapes, Blending with theCrosslinker-Accelerator System for Thermal Crosslinking, and Coating:

The acrylate hotmelt PSAs prepared by Processes 1-2 were melted in afeeder extruder (single-screw conveying extruder from Troester GmbH &Co. KG, Germany) and using this extruder were conveyed as a polymer meltinto a twin-screw extruder (Leistritz, Germany, ref. LSM 30/34). Theassembly is heated electrically from the outside and is air-cooled by anumber of fans, and is designed such that, with effective distributionof the crosslinker-accelerator system in the polymer matrix, there is atthe same time a short residence time ensured for the adhesive in theextruder. For this purpose the mixing shafts of the twin-screw extruderwere arranged in such a way that conveying elements are in alternationwith mixing elements. The addition of the respective crosslinkers andaccelerators is made with suitable metering equipment, where appropriateat two or more points (FIG. 1: metering points 1.1 and 1.2) and, whereappropriate, with the use of metering assistants into the unpressurizedconveying zones of the twin-screw extruder. Following exit of theready-compounded adhesive, i.e. of the adhesive blended with thecrosslinker-accelerator system, from the twin-screw extruder (exit:circular die, 5 mm diameter), coating takes place in accordance withFIG. 1 onto a carrier material in web form.

The time between metered addition of the crosslinker-accelerator systemand the shaping or coating procedure is termed the processing life. Theprocessing life indicates the period within which the adhesive, blendedwith the crosslinker-accelerator system, or the viscoelastic carrierlayer, can be coated with a visually good appearance (gel-free,speck-free). Coating takes place with web speeds between 1 m/min and 20m/min; the doctor roll of the 2-roll applicator is not driven.

In the examples and tables below, the formulations employed, theproduction parameters and the properties obtained are each described inmore detail.

Process 4: Preparation of the Modified Hotmelt PSAs and ViscoelasticCarriers:

The base polymer P3 was blended with 0.2% by weight of Uvacure® 1500,based on the polymer, then diluted with acetone to a solids content of30% and subsequently coated from solution onto a siliconized releasefilm (50 μm polyester) and onto a 23 μm etched PET film (coating speedof 2.5 m/min, drying tunnel 15 m, temperatures: Zone 1: 40° C., Zone 2:70° C., Zone 3: 95° C., Zone 4: 105° C.). The coat weight was 50 g/m².

Examples B1 to B4

The base polymer P1 was polymerized in accordance with thepolymerization process described, concentrated in accordance withProcess 1 (solids content 99.8%) and then blended with the Dertophene®T105 resin in accordance with Process 2. These resin-modified acrylatehotmelt compositions were then compounded in accordance with Process 3continuously with the crosslinker-accelerator system consisting of a

-   -   pentaerythrityl tetraglycidyl ether,    -   here Polypox® R16 from UPPC AG (epoxide) and    -   the respective accelerator.

Detailed description: In the twin-screw extruder described in Process 3,a total mass flow consisting of 70 parts of polymer P1 and 30 parts eachtime of Dertophene® T105 resin of 533.3 g/min (corresponding to 373grams of the pure polymer per minute) was blended with 0.70 g/min of theepoxide crosslinker pentaerythritol tetraglycidyl ether (correspondingto 0.19% by weight based on polymer) and with a predetermined amount ofaccelerator. The amount of accelerator was selected such that the numberof activating basic groups is constant (see Table 1). Because of varyingnumbers of functionalities per molecule, there is thus a variation inthe molar concentration of the accelerator based on the polymer.

The epoxide was metered via a peristaltic pump at metering point 1.1,and the amine was metered separately via a peristaltic pump at meteringpump 1.2 (see FIG. 1). To improve meterability and the quality of mixingachievable, the crosslinker system used was diluted with the liquiddimethyl propylphosphonate Levagard® DMPP from Lanxess (ratio of thecrosslinker 0.5:1). The operational parameters are summarized in Table2.

The processing life of the completed compounded formulations was morethan 7 minutes with an average composition temperature of 125° C. afterdeparture from the Leistritz twin-screw extruder. Coating took place ona 2-roll applicator in accordance with FIG. 1, at roll surfacetemperatures of 100° C. in each case. The coat weight in each case was50 g/m² on 23 μm PET film. On the adhesive tapes thus produced,measurements were made of the bond strength to steel at room temperatureand microshear travel at 40° C. as a function of the storage time(selected example are shown in FIG. 5). The technical adhesive data ofExamples B1 to B4 are summarized in Table 3. These examples show thatvery high-performance adhesive tapes can be produced, featuring, amongother qualities, high bond strengths to polar and apolar substrates(steel and polyethylene) and good cohesive properties even under theinfluence of temperature.

TABLE 1 Accelerator concentrations Amine Concentration groups/100 gExample Accelerator [%] polymer [mol] Example B1 N,N,N,N,N- 0.75 1.2(inventive) pentamethyldiethylene- triamine Example B2 N′-(3- 0.7 1.2(inventive) (dimethylamino)propyl)- N,N-dimethyl- 1,3- propanediamineComparative diethylenetriamine 0.41 1.2 example B3 Comparative N,N,N,N-0.61 1.2 example B4 tetramethylmethane diamine

When the crosslinker-accelerator system of the invention is used, thecrosslinking reaction proceeds to completion via the functional groupsof the polyacrylate, even without supply of heat, under standardconditions (room temperature). Generally speaking, after a storage timeof 7 days to 14 days, the crosslinking reaction has concluded to anextent such that an adhesive tape or carrier layer present isfunctional. The ultimate crosslinking state and hence the ultimatecohesion of the composition are achieved, depending on the choice of thecomposition/crosslinker system, after a storage time of 14 to 30 days,in advantageous form after 14 to 21 days' storage time at roomtemperature, expected to be earlier in the case of a higher storagetemperature.

As a result of the crosslinking there is an increase in the cohesion ofthe adhesive and hence also in the shear strength. The linking groupsobtained are very stable. This allows very ageing-stable andheat-resistant self-adhesive tapes. It can be shown, moreover, that thechoice of accelerator barely influences the adhesive properties, butvery strongly influences the room temperature kinetics (see FIG. 5 andTable 3). Consideration of Comparative Examples B3 and B4 shows that thecrosslinking is not yet complete within the abovementioned period, orthat gelation occurs beforehand in the process unless the inventiveaccelerator-crosslinker system is used.

Examples Viscoelastic Carriers and Three-Layer Constructions Process 5:Production of the 3-Layer Constructions by 2-Roll Calender:

The base polymer P2 was freed in the same way as in Process 1 from thesolvents, and then optionally was admixed with additives in the same wayas in Process 2. Thereafter the process was carried out as described inFIG. 4. Using a distributor nozzle (1), the viscoelastic composition(3), already compounded with the crosslinker-accelerator system and withfillers where appropriate, is supplied to the roll nip. The shaping ofthe viscoelastic composition to a viscoelastic film takes place betweenthe calender rolls (W1) and (W2) in the roll nip between twoself-adhesive compositions (6a, 6b), which in turn are supplied in aform where they are coated on anti-adhesively furnished carriermaterials (5a, 5b). At the same time, the viscoelastic composition isshaped to the set layer thickness, and is coated with the twoself-adhesive compositions supplied. In order to improve the anchoringof the self-adhesive compositions (6a, 6b) on the shaped, viscoelasticcarrier layer (4), the self-adhesive compositions are corona-treated bycorona station (8) (corona unit from Vitaphone, Denmark, 100 W·min/m²)before being supplied to the roll nip. After the three-layer assemblyhas been produced, this treatment leads to improved chemical attachmentto the viscoelastic carrier layer.

The web speed when travelling through the coating unit is 30 m/min.

Following departure from the roll nip, any anti-adhesive carrier (5a) isremoved, and the completed three-layer product (9) is wound up with theremaining, second anti-adhesive carrier (5b).

Specific examples for the production of the self-adhesive compositionsand the coating of the inventive adhesive tapes are presented below.

Examples MT1 to MT2

The base polymer P2 was concentrated by Process 1 (solids content 99.7%)and then compounded by Process 3 in a twin-screw extruder continuouslywith

MT1: the crosslinker-accelerator system consisting of

-   -   pentaerythritol tetraglycidyl ether (Polypox® R 16; 0.14% by        weight based on the polyacrylate)        and    -   N′-(3-(dimethylamino)propyl)-N,N-dimethyl-1,3-propanediamine        (Jeffcat® Z-130; 0.14% by weight based on the polyacrylate);        MT2: the crosslinker-accelerator system consisting of    -   pentaerythritol tetraglycidyl ether (Polypox® R 16; 0.14% by        weight based on the polyacrylate),    -   N′-(3-(dimethylamino)propyl)-N,N-dimethyl-1,3-propanediamine        (Jeffcat® Z-130; 0.14% by weight based on the polyacrylate)        and also    -   Expancel® 051 DU 40 (expandable hollow thermoplastic microbeads,        0.70% by weight based on the polymer).

Coating to produce the viscoelastic carriers VT1 and VT2 from the basepolymer P2 between the composition layers P3, coated beforehandaccording to Process 4 onto siliconized polyester films, takes place ona 2-roll applicator at roll temperatures of 100° C. by Process 5. Thelayer thicknesses of the viscoelastic carriers VT1 and VT2 was 800 μm.The corona power was 100 W·min/m². After 7 days of room-temperaturestorage, the technical adhesive data were measured for both the openside and the lined side. The data of Examples MT1 and MT2 is summarizedin Table 4.

As can be seen from the data in Table 4, the double-sided adhesiveassembly tapes of the invention have very good technical adhesive data.Particularly positive is the balanced adhesive profile of the respectivesides. With the same layer of adhesive on both sides of the adhesivetape, these sides exhibit virtually the same technical adhesive data.This shows the homogeneous crosslinking through the layer. Moreover,these three-layer adhesive tapes do not exhibit any delamination. Byvirtue of the corona treatment of the pressure-sensitive adhesive layersand of the secondary crosslinking of the adjacent viscoelastic carrierlayer, the anchoring of the layers to one another is very good.

TABLE 2 Process parameters Process parameters Total mass Target currentTSE Melt temp. Base polymer Compounding as per through- TSE consumptionoutlet after Doctor Coating Process- K process 2 put in TSE speed of TSEpressure TSE roll roll ing time Example Polymer value Fraction ofadjuvants [kg/h] [1/min] [A] [bar] [° C.] DR CR [min] B1-B4 P3 75.1 70parts polymer P3 + 32.0 110 15 12 125 100 100 greater 30 parts DT 105resin than 7 TSE = Twin-screw extruder; DT 105 = Dertophene ® T105

TABLE 3 Adhesive results Adhesive properties after storage of thespecimens at room temperature for 25 days Bond Bond Holding Holding MSW40° C./ Base polymer Coat- strength strength power 10N, power 10N,elast. K Compounding process 2 Carrier weight to steel to PE 23° C. 70°C. fraction Example Polymer value Fraction of adjuvants film [g/m²][N/cm] [N/cm] [min] [min] [μm]/[%] B1 P3 75.1 68 parts polymer P3 + 23μm 50 8.5 3.8 1.118 95 563/64 32 parts DT 105 resin PET sheet B2 P3 75.168 parts polymer P3 + 23 μm 50 7.9 4.2 1.437 105 491/60 32 parts DT 105resin PET sheet B3 P3 75.1 68 parts polymer P3 + 23 μm 50 Tests notpossible, — (comp.) 32 parts DT 105 resin PET formulation has gelatedsheet B4 P3 75.1 68 parts polymer P3 + 23 μm 50 Tests not possible,greater than (comp.) 32 parts DT 105 resin PET formulation has notcrosslinked 2000/0 sheet Bond strength steel/PE = measurement method H1Holding power = measurement method H2 MST = Microshear travel =measurement method H3 DT 105 = Dertophene ® T105 For the acceleratortype see Table 2

TABLE 4 Product construction and technical adhesive data for thethree-layer constructions Bond strength to steel Holding powerThree-layer product Carrier [N/cm] 10N 23° C. [min] Viscoelasticthickness open lined open lined Density Example PSA 1 carrier layer PSA2 [μm] side side side side [g/cm³] MT1 50 g/m² VT 1 50 g/m² 800 24.223.7 >10 000 >10 000 1.08 P2 P3 MT2 50 g/m² VT 2 50 g/m² 800 35.2 34.6   7850    6970 0.78 P2 P3 Bond strength to steel = measurement methodV1 Holding power = measurement method V2

1. A crosslinker-accelerator system for the thermal crosslinking ofpolyacrylates having functional groups suitable for entering intolinking reactions with epoxide groups, comprising a crosslinkercomprising at least one substance comprising at least one epoxide group;and an accelerator comprising at least one substance conforming to thegeneral formula (I)R¹R²N—CR³R⁴—CR⁵R⁶—(CR⁷R⁸)_(n)—NR⁹R¹⁰  (I) wherein the radicals R¹, R²,R⁹ and R¹⁰ independently of one another are a hydrogen atom or asubstituted or unsubstituted alkyl or cycloalkyl radical having 1 to 8carbon atoms or an alkylene group bonded to the main chain and having 1to 8 carbon atoms, where at least one of the radicals R¹, R², R⁹ and R¹⁰is not a hydrogen atom; the radicals R³, R⁴, R⁵, R⁶, R⁷ and R⁸independently of one another are a hydrogen atom or an alkyl grouphaving 1 to 8 carbon atoms or form a 5-7-membered cycloalkylene group;and n is an integer from 0 to
 4. 2. The crosslinker-accelerator systemaccording to claim 1, wherein at least one of the radicals R¹, R², R⁹and R¹⁰ is a methyl group.
 3. The crosslinker-accelerator systemaccording to claim 1 wherein R¹ and R² and/or R⁹ and R¹⁰ are each amethyl group.
 4. The crosslinker-accelerator system according to claim 1wherein n is 0 (zero).
 5. The crosslinker-accelerator system accordingto claim 1 wherein n is 0 (zero) and R¹ and R² and/or R⁹ and R¹⁰ areeach a methyl group.
 6. The crosslinker-accelerator system according toclaim 1 wherein the ratio of the number of all the substituted andunsubstituted amino and phosphine groups in the accelerator to thenumber of epoxide groups in the crosslinker is from 0.2:1 to 4:1.
 7. Athermally crosslinkable composition comprising at least one polyacrylatehaving functional groups suitable for entering into linking reactionswith epoxide groups, and a crosslinker-accelerator system according toclaim
 1. 8. The thermally crosslinkable composition according to claim7, wherein the ratio of the total number of epoxide groups in thecrosslinker to the number of functional groups in the polyacrylate thatare suitable for entering into linking reactions with epoxide groups isin the range from 0.01:1 to 1:1.
 9. The thermally crosslinkablecomposition according to claim 7 wherein the total fraction ofcrosslinker is 0.1%-5% by weight and in that the total fraction ofaccelerator is 0.05%-5% by weight, based in each case on the purepolyacrylate to be crosslinked.
 10. A crosslinked polyacrylateobtainable by thermal crosslinking of a thermally crosslinkablecomposition according to claim
 7. 11. Method for the thermalcrosslinking of polyacrylates having functional groups suitable forentering into linking reactions with epoxide groups, comprising the useof a crosslinker-accelerator system according to claim
 1. 12. (canceled)13. (canceled)